Signals may be modulated using phase-shift keying (PSK) and amplitude-shift keying (ASK) modulation. In PSK modulation, changes in the phase of a signal are used to represent data, and in ASK modulation, the amplitude levels of a signal are used to represent data.
PSK-ASK communication, however, may experience difficulties. As an example, PSK-ASK communication typically uses a non-optimal extinction ratio. An extinction ratio refers to the ratio of the optical power of a “1” pulse to that of a “0” pulse. In general, a higher ASK extinction ratio improves ASK communication, but degrades PSK communication. For example, ASK communication may be optimal at an extinction ratio of 10 decibels, but PSK communication may not be optimal. Typically, a compromise extinction ratio between the optimal extinction ratio for ASK communication and the optimal extinction ratio for PSK communication is used. For example, an extinction ratio of 7 decibels may be used. The compromise extinction ratio, however, is typically not optimal for either ASK or PSK communication.
As another example, PSK-ASK communication may suffer from crosstalk between the ASK and PSK signals. The ASK signal may induce a non-linear phase shift on the PSK signal. The non-linear phase shift introduces non-linear phase noise to the PSK signal, degrading the PSK signal. In addition, self-phase modulation may induce a phase shift that introduces a frequency chirp, which may degrade the ASK signal.